Steel pipes used as oil country tubular goods or line pipes are broadly divided into welded steel pipes (for example, electric resistance welded steel pipes, UOE steel pipes, and other pipes) and seamless steel pipes. Among these steel pipes, electric resistance welded steel pipes are advantageous from an economical point of view because they can be manufactured at a low cost by using steel strips formed by hot rolling (so called hot rolled steel coils) as a starting material.
However, a typical electric resistance welded steel pipe inevitably includes a joint (a so-called seam) caused by welding because the electric resistance welded steel pipe is manufactured by forming a steel strip into a cylindrical shape using forming rolls to form an open pipe (here, an open pipe refers to a pipe-shaped strip formed by multiple forming rolls and having unwelded edges and such a pipe is referred to as an open pipe, below) and performing electric resistance welding (also referred to as high-frequency resistance welding) on edges of the open pipe (i.e., both side edges of the cylindrically formed steel strip) while pressing the open pipe with squeeze rolls. Such a seam has a problem of poor low-temperature toughness. Thus, oil country tubular goods or a line pipe employing an electric resistance welded steel pipe is difficult to use in a cold district. The reason why such a seam has poor low-temperature toughness is because, while edges are being welded together, the high-temperature molten metal reacts with atmospheric oxygen and forms an oxide, which is likely to remain in the seam.
The electric resistance welded steel pipe has another problem in that the corrosion resistance of the seam is likely to be reduced because an alloy element in the molten metal is likely to segregate while the edges are being welded together. Thus, oil country tubular goods or line pipes employing electric resistance welded steel pipes are difficult to use in a severe corrosion environment (for example, sour environment).
Meanwhile, welding with laser beams (hereinafter referred to as laser welding) has been receiving attention as a welding method with which the low-temperature toughness and the corrosion resistance of the seam are not reduced. With the laser welding, a heat source can have a small size and high-density heat energy can be converged. The laser welding thus can prevent an oxide from being formed in the molten metal or an alloy element from segregating in the molten metal. Thus, when the laser welding is employed in the manufacturing of welded steel pipes, the low-temperature toughness and the corrosion resistance of the seam can be prevented from being reduced.
Thus, a technology for manufacturing steel pipes (i.e., laser welded steel pipes) by emitting laser beams onto the edges of open pipes to weld the edges together has been put into practice during manufacturing of welded steel pipes.
The laser welding, however, is performed by irradiating a weld portion with a laser beam, which is a high-energy density light beam, converged by an optical component and causes a metal to melt rapidly. Thus, the molten metal scatters from the formed molten weld pool as spatter. The scattered spatter adheres to welding equipment, thereby degrading the quality of the seam. Concurrently, the spatter also adheres to the optical component, making the welding operation unstable. Since laser welding is performed by converging high-density heat energy, a large amount of spatter is formed, causing welding defects such as occurrence of undercut or underfill (or depression). If underfill occurs in a weld portion, the strength of the weld portion is reduced.
In view of these problems, various technologies for preventing adherence of spatter or occurrence of spattering during laser welding have been studied. For example, a technology for preventing the occurrence of spattering by reducing laser power or by changing a focus position by a large degree (that is, by defocusing) has been put into practice. Reduction of laser power or defocusing, however, not only leads to reduction of welding speed (i.e., reduction of weld efficiency) but also involves a problem of making lack of penetration more likely to occur.
Patent Literature 1 discloses a technology for preventing occurrence of spattering by dividing a laser beam into multiple laser beam spots. However, the laser welding technology using multiple laser beam spots into which a laser beam is divided is similar to the laser welding technology with a lower laser power. Thus, such a technology involves not only reduction of weld efficiency but also a problem of making lack of penetration more likely to occur. Furthermore, since prisms that divide laser beams are expensive, the cost of the welding operation inevitably increases.
Patent Literature 2 discloses a technology for preventing underfill of deposit metal from occurring by using a filler wire during laser welding. With this technology, however, the components of the filler wire change the composition of the weld metal. Thus, an appropriate filler wire has to be selected in accordance with the components of open pipes, thereby increasing the burden of stock management of filler wires or production control of laser welding.
Patent Literature 3 discloses a technology for preventing welding defects by using laser welding and arc welding together. This technology, however, makes the structure of welding equipment complex, thereby increasing the burden of not only maintenance but also management of the welding operation.
Patent Literature 4 discloses a method of using two circular beam spots. With this technology, however, welding defects are not reduced during laser welding performed under conditions in which stress occurs in a weld portion (under conditions such as where stress occurs in a molten weld pool due to the weld portion of an electric resistance welded steel pipe receiving an upset). Particularly, the amount of spatter generated from the back surface of a steel strip increases.